1. Statement of the Invention
This invention relates to the concentration of alkaline bismuthide in a material comprising alkaline bismuthide and lead, and more particularly to a new and improved method for the concentration of alkaline bismuthide in a dross comprising alkaline bismuthide and lead.
2. Description of the Prior Art
It is known in the prior art to remove bismuth from lead by treating the molten bath of bismuth-containing lead with appropriate alkaline earth or alkali metal as debismuthizing reagent. The bismuth forms an alkaline bismuthide with the debismuthizing reagent and the alkaline bismuthide forms a dross on the molten bath surface, the dross being referred to in the art as alkaline bismuthide dross or as "debismuthizing" dross. The dross contains an appreciable amount of lead in addition to the alkaline bismuthide. Calcium and magnesium are presently utilized as debismuthizing reagents for removing the bismuth from the lead. Sodium and potassium have also been utilized heretofor as the debismuthizing reagent. The dross is then separated from the lead, for instance by skimming.
In the recovery of the bismuth from the debismuthizing dross, it is important that the alkaline bismuthide be concentrated in the dross in preparation for further refining to obtain pure bismuth. In this regard, it would be highly desirable to at least double the bismuth content of the debismuthizing dross in preparation for further refining. For example, if the debismuthizing dross contained 6% of bismuth as alkaline bismuthide, it is highly desirable to concentrate the bismuthide in the dross so as to increase the bismuth content to 12% or higher.
Heretofor the bismuthide has been concentrated in the debismuthizing dross by either mechanically pressing the dross, or by liquating the dross under a flux. In the pressing of the debismuthizing dross, the dross was pressed to expel molten lead from the bismuthide particles by steel plates by means of hydraulic pressure applied to the plates. Some molten lead is expelled but the pores of the alkaline bismuthide solid mass or cake are soon blocked due to significant physical compression of the bismuthide cake by reason of the mechanical pressing. Consequently, additional lead can not be pressed out of the cake to any considerable extent, and the cake retains considerable molten lead occluded within the bismuthide cake, with molten lead surrounding partially or completely bismuthide particles in the interior of the cake. Further, the interior of the compressed cake is substantially impermeable or non-pervious to air due to the mechanical pressing, and it is necessary that alkaline bismuthide cake be air permeable or pervious for a subsequent bismuth recovery operation involving heating of the alkaline bismuthide cake in air, to ignite and burn autogenously the cake to liberate elemental bismuth from the alkaline bismuthide. In the liquating method, the debismuthizing dross is liquated under a flux, usually a chloride flux. Liquating the dross under a flux is costly for the reasons considerable bismuth is diverted back to the lead and the chloride flux is expensive. Further, liquating under a flux presents a problem of disposal of slag produced, for the reason a salt slag is produced that is usually difficult to dispose of. Moreover, the liquation of the dross under a flux is a high temperature operation, involving temperatures of about 700.degree. C.